In fact it has been common practice to use Heavy Fuels Oils as a dumping ground for all sorts of waste products. With some legitimacy refiners have been know to use their fuel oil as a disposal medium for "mistakes", products that are not economically recoverable, and tend to not always use commercial grades but whatever is convenient.
However, the new ISO 8217 fuel standard (visit the IBIA and CIMAC web sites for details from their fuels working groups) has a new set of limits specifically designed to protect fuel oil quality from unprocessed waste lubricating oils.
One of the main problems is that these fuels are as often used for marine diesels and the contaminants in waste oils can be both very dmaging to the engines and can present a health hazard to operators.
It is to be hoped that the new MARPOL Annex VI legislation will ultimately result in much better control of fuel oil quality. Only 25-30% of bunkers are said to be sampled. This is an open door for all sorts of abuses. MARPOL should require or result in all bunkers being sampled and analysed and a commercial analysis will specifically test for used lubricant addittion through the characteristic contaminants.
However, from your question it is implicit that this "dumping" would take place into the settling tanks by-passing the fuel quality checks. I have to suspect that any adulteration of the fuel once it has been bunkered, will be frowned on by the authorities. Under MARPOL it is expected that XRF (X-Ray Fluorescence sulphur in fuel) analysers will be installed at the engines , probably sampling from the high pressure circuit. I anticipate that when XRF is mandated there will also be a requirement to sample from the high pressure circuit (from the mixing tank, perhaps, as now). SOme engine manufacturers already use the fuel heater viscometer for fuel quality checking (the newer digital viscometers) because they can use this data to identify the fuels used from the fuel analysis records. Adulterating the fuel in the settling tanks will probably affect this measurement and could lead to engine warranty problems.
I would suggest that the engine manufacturers are consulted and i would suggest finding out the preferred means of disposing of used lubricants. Most often they are re-processed or cleaned before disposal.
